Many industrial processes require the use of optical-based measurement instrumentation for obtaining data regarding size distribution of granular matter in the form of particles, fragments or flakes that are to be handled in bulk during their processing, such as ore processed in mining industry, wood chips used to produce pulp and paper and mill waste matter used in the manufacturing of fibreboards. Uniform chip size is very important for the production of high quality pulp. Changes in the distribution of sizes influence chip bulk density under a dynamic feeding condition, and therefore influence the specific energy applied at the refining stage. Oversize chips require more energy and produce poor pulp quality, while fines and pin chips decrease the pulp strength. Several types of chip classifiers are available for off-line laboratory testing, and few systems have been proposed to perform on-line discontinued measurements, as discussed by Bergman, T. in “On-line Chip Analysis: New Technology for an Improved Pulping Process”, Pulp & Paper Canada, (12) 150-151 (1999). These measurements took only one portion of the wood chips for evaluating the size distribution of all chips, and were not really representative. For example, the well known Williams classifier makes use of superposed sifting trays of decreasing perforations sizes (Typically: ⅛<size<⅜ in.; ⅜<size<⅝ in.; ⅝<size<⅞ in.; ⅞<size<1⅛ in.; size>1⅛ in.) to physically separate the wood chips of a test sample according to increasing grain sizes, thus producing a plurality of sub-samples that are subsequently weighted to obtain a weight distribution in function of grain size classes. Chip size classification using a chip classifier such as Williams is frequently performed at chip reception sites of mills, but this offline measurement cannot be conveniently used to stabilize and control a TMP process. In order to perform an on-line chip size distribution measurement, a computerized grain size measurement method using image processing technique is preferably used, such as disclosed in U.S. Pat. No. 7,292,949 issued to the same assignee as of the present patent application. The process of computing grain size is similar to sifting sand through a screen. By gradually increasing the screen size, only the larger sand grains will be left at the end of the sifting process. As a result, the number and weight of sand grains can be plotted as a function of screen size. The resulting curve represents the grain size distribution of the particles in the tested sample. While such on-line computerized grain size measurement technique presents advantages compared with off-line Williams classifier, its accuracy has proved to be limited.